As dimensions of semiconductor devices continue to decrease and packaging densities continue to increase, parasitic capacitance becomes more and more important to improve the operating speed of devices (e.g., transistors). For example, a gate structure of a transistor usually includes one or more sidewall spacers positioned adjacent to the gate structure. The sidewall spacers are used to define the gate cavity in a replacement gate manufacturing process and to protect a final replacement gate structure. Oftentimes, the sidewall spacers are made of silicon nitride which has a relatively high dielectric constant value (i.e., a high k value) of, e.g., about 7-8. As a result of the transistor structure, the high-k silicon nitride spacers usually tend to increase the parasitic capacitance between the gate electrode and self-alighted contacts on source/drain regions of the transistor, which often reduces the switching speed of the transistor. Thus, low-k dielectric materials (e.g., k<6) are sometimes implemented to fabricate sidewall spacers in order to improve the transistor performance.